1. Field of the Invention
The present invention relates to a multicolor image forming apparatus, and more particularly to a multicolor image forming apparatus such as a color laser printer and a color digital copying machine of a multi-photoreceptor system that obtains an image by scanning light beams onto a plurality of photoreceptors.
2. Description of the Related Art
As a conventional image forming apparatus of a multi-photoreceptor system, the one having a structure as shown in FIG. 23 is proposed (see Japanese Published Unexamined Patent Application No. Hei 3-142412).
In such an image forming apparatus, generally, laser beams emitted from four laser light sources corresponding to cyan (C), magenta (M), yellow (Y), and black (K) are bidirectionally scanned by a polygon mirror 201 mounted in a light deflector. Thereafter, the laser beams pass through a scanning imaging optical system, are reflected in mirrors 202 to 213, and scan on the photoreceptors 214 to 217 corresponding to the colors. Latent images formed on the photoreceptors are developed by a developing apparatus and successively transferred onto paper transported by a transporting belt 218 to form a color image.
Another conventional image forming apparatus will be described with reference to FIGS. 25 to 27. In these drawings, the reference numeral 250 designates an exposing apparatus; 252K, 252Y, 252M, and 252C, laser beams; 254, a deflector; 256K, 256Y, 256M, and 256C, mirror assemblies; 258K, 258Y, 258M, and 258C, photoreceptors; 260A and 260B, registermarks; and 262A and 262B, detecting parts. In the exposing apparatus 250, the four laser beams are scanned, by one deflector 254, on the photoreceptors 258K, 258Y, 258M, and 258C corresponding to black (K), yellow (Y), magenta (M), and cyan (C), a transfer material (paper) not shown is transported by a belt 264, and multiple images are transferred from the photoreceptors onto the transfer material, whereby a color image is formed. The exposing apparatus 250 scans and deflects, by the deflector 254, laserbeams 252K, 252Y, 252M, and 252C containing information, emitted from light sources (not show), which are irradiated straightly onto the materials 258K, 258Y, 258M, and 258C to be scanned after passing through imaging lens systems (not shown) and mirror assemblies 256K, 256Y, 256M, and 256C into which reflecting mirrors are built. SOS (Start of Scan) is used to determine a write timing of the laser beams 252K, 252Y, 252M, and 252C, which are scanned and irradiated onto the photoreceptors 258K, 258Y, 258M, and 258C with a given delay after passing through a pickup mirror 266 and being detected by synchronous light detecting apparatuses 268K, 268Y, 268M, and 268C. To correct a skew of each color, the gradient of a scanning line (hereinafter referred to as a skew) is adjusted in this case. For example, the mirror assemblies 256K, 256Y, 256M, and 256C are provided at one end with a rotation center 270 and at the other with an actuator 272, register marks 260A and 260B are formed on the belt 264 and are detected by the detecting parts 262A and 262B to detect skews of scanning lines, and based on the result, the actuator 272 is driven up and down in the E direction around the rotation center 270, whereby skew adjustments are performed to form a color image free from color skews. Since the rotation center 270 is provided around an axis intersecting a light path of the above described SOS light, such a configuration prevents the SOS light from deviating from the synchronous light detecting apparatuses 268K, 268Y, 268M, and 268C due to the rotation of the mirror assemblies 256K, 256Y, 256M, and 256C during skew adjustments. Such an exposing apparatus 250 is disclosed in, e.g., Japanese Published Unexamined Patent Application No. Hei 3-150521.
Further, the configuration of another conventional exposing apparatus 450 will be described with reference to FIGS. 28 to 30. In the exposing apparatus 450, a laser beam 454 is emitted from a light source 452, passes through a light-gathering lens not assigned a reference numeral, is deflected by a deflector 456, passes through an imaging lens system 458, is reflected by a reflecting mirror 462 of a mirror assembly 460, and is irradiated onto a photoreceptor 464. The mirror assembly 460 is provided with a rotation center 466 in an upper portion near the central portion of scanning, and an adjusting part 468 and a spring 470 are provided at one end of the mirror assembly. In such a configuration, since the mirror assembly 460 rotates around the upper portion near the central portion of scanning during skew adjustments by moving the adjusting unit along the direction of the arrow D, skews can be adjusted. Such a mirror assembly configuration of an exposing apparatus is disclosed in, e.g., Japanese Published Unexamined Patent Application No. Sho 59-7331.
A disclosed method of correcting axis skews of photoreceptors due to a mounting error occurring when the photoreceptors 214 to 217 are mounted in such an image forming apparatus has problems of the complicated mechanism and high cost. In this method, a register mark 219 transferred onto a transporting belt 218 is sensed by a sensor such as CCD220, and mirrors 202 and 203 as shown in FIG. 24 are moved by actuators 223 to 225. Also, as technology to simplify mechanism, a method is proposed which corrects axis skews of photoreceptors by operating optical parts within an exposing apparatus from the outside. However, in an exposing apparatus that scans scanning beams on both photoreceptors and a polygon mirror, as shown in FIG. 23, since the directions of scanning lines 226 to 229 scanned in both of them are opposite such as the direction of the arrow A (scanning lines 226 and 227) and the direction of the arrow B (scanning lines 228 and 229), adjusting parts are placed separately in the front and rear parts of an image forming apparatus, posing a problem that a large number of man-hours are needed for adjustment operations. Also, for safety purposes and from a configuration viewpoint, adjustment results are not obtained until an image forming apparatus has been assembled, requiring repetitive operations such as many disassembling and assembling operations and confirmation of image color skew amounts.
Furthermore, since the exposing apparatus 250 shown in FIGS. 25 and 26 is constructed to scan four laser beams by one deflector 254, as shown in FIG. 27, the synchronous light detecting apparatuses 268K, 268Y, 268M, and 268C are placed in a staggered manner across the deflector 254. In this example, a maintenance space need not be considered so seriously because the actuator 272 is small and skew adjustments are automatically performed. However, in order to reduce costs, if the actuator 272 is displaced so as to manually move relevant parts and skew dials 274K, 274Y, 276M, and 274C are provided as shown in FIG. 27, since a maintenance space (a space in which to manually move the dials) must be provided, there arises a problem that the size of a multicolor image forming apparatus increases by twice S.
A mirror assembly 460 of further another conventional exposing apparatus 450 as shown in FIGS. 28 and 29 has a problem that, if skew adjustments are performed as shown in FIG. 30, a laser beam 454C passing on the rotation center 466 is unchanged in light path length, but a laser beam 454E passing through a scanning end portion changes by X' in light path length, from X+Y to X+X'+Y.